My 1500 sq ft house is currently 1 zone. There are two loops off the boiler (left and right side of the house - it's a Cape, so it's pretty symmetrical). The 10 radiators on the first floor (5 on each loop) are each fed by a pair of monoflow tees.

The second floor's heating is 2 x 15' baseboards, one on each side of the house.
These baseboards are supplied by a monoflow T pair, from each of the 1st floor loops; so the second floor supply/return appear no different than all other 1st floor radiators.

I want the 2nd floor to be on its own zone. Would there be any problems by moving the 2nd floor's supply to its own zone, but a series loop, while the first floor remains all on monoflow tees?

This would be a project for when it's 60 degrees out, not -10!!

 

D,
The short answer for now is yes.Remove the second floor baseboards from the monoflo loop and put it on its own loop. There are obviously other components needed but it can be done and is a good idea.